iOS 14 Widgets: How to create different layouts for every widget size family?

The WidgetFamily (Apple Documentation) enum as part of WidgetKit will allow you to switch upon the various sizes within your view and adjust accordingly. Set this as an @Environment variable and switch on the avaliable cases:

• .systemSmall
• .systemMedium
• .systemLarge

struct WidgetView : View {
   @Environment(\.widgetFamily) var family

    @ViewBuilder
    var body: some View {
        
        switch family {
        case .systemSmall:
            Text("Small")
        case .systemMedium:
            Text("Medium")
        case .systemLarge:
            Text("Large")
        default:
            Text("Some other WidgetFamily in the future.")
        }

    }
}

Additionally to the accepted answer, in your Provider class methods (getTimeline, getSnapshot & placeholder) you get a context object which has a family member var.

family can be one of the three widget sizes: .systemSmall, .systemMedium & .systemLarge

Apple's official documentation.